Swift 4.x (x < 2) names are obsoleted in 4.2, not 5.

In APINotes, when an unversioned name overrides
a previously declared name with a Swift version, if the
previously declared name is guarded under version 4
the obsoleted version should be 4.2, not 5.  In all other
cases, the obsoleted version is the next major Swift
version.

Fixes rdar://problem/39950937

lib/ClangImporter/ImportDecl.cpp

@@ -2359,12 +2359,20 @@ namespace {
             PlatformAgnosticAvailabilityKind::UnavailableInSwift);
       } else {
         unsigned majorVersion = getVersion().majorVersionNumber();
+        unsigned minorVersion = getVersion().minorVersionNumber();
         if (getVersion() < getActiveSwiftVersion()) {
           // A Swift 2 name, for example, was obsoleted in Swift 3.
+          // However, a Swift 4 name is obsoleted in Swift 4.2.
+          // FIXME: it would be better to have a unified place
+          // to represent Swift versions for API versioning.
+          clang::VersionTuple obsoletedVersion =
+            (majorVersion == 4 && minorVersion < 2)
+                ? clang::VersionTuple(4, 2)
+                : clang::VersionTuple(majorVersion + 1);
           attr = AvailableAttr::createPlatformAgnostic(
               ctx, /*Message*/StringRef(), ctx.AllocateCopy(renamed.str()),
               PlatformAgnosticAvailabilityKind::SwiftVersionSpecific,
-              clang::VersionTuple(majorVersion + 1));
+              obsoletedVersion);
         } else {
           // Future names are introduced in their future version.
           assert(getVersion() > getActiveSwiftVersion());

test/APINotes/Inputs/custom-modules/ObsoletedAPINotesTest.apinotes

@@ -0,0 +1,10 @@
+Name: M
+Classes:
+- Name: FooID
+  SwiftName: Foo_ID
+
+SwiftVersions:
+- Version: 4
+  Classes:
+  - Name: FooID
+    SwiftName: FooID

test/APINotes/Inputs/custom-modules/ObsoletedAPINotesTest.h

@@ -0,0 +1,3 @@
+@import Foundation;
+@interface FooID: NSObject
+@end

test/APINotes/Inputs/custom-modules/module.modulemap

@@ -1,3 +1,6 @@
 module APINotesTest {
   header "APINotesTest.h"
 }
+module ObsoletedAPINotesTest {
+  header "ObsoletedAPINotesTest.h"
+}

test/APINotes/obsoleted.swift

@@ -0,0 +1,7 @@
+// RUN: not %target-swift-frontend -typecheck -verify -I %S/Inputs/custom-modules -F %S/Inputs/custom-frameworks -swift-version 3 %s
+// RUN: %target-swift-frontend -typecheck -verify -I %S/Inputs/custom-modules -F %S/Inputs/custom-frameworks -swift-version 4.2 %s
+// RUN: %target-swift-frontend -typecheck -verify -I %S/Inputs/custom-modules -F %S/Inputs/custom-frameworks -swift-version 5 %s
+import ObsoletedAPINotesTest
+
+let _: FooID // expected-error{{'FooID' has been renamed to 'Foo_ID'}}
+let _: Foo_ID